According to NBC Philadelphia, a dozen people have already been rescued from the site on 22nd and Market Streets, with at least two people taken away on stretchers. Firefighters and emergency personnel are currently searching for survivors.

NBC is reporting that an adjacent building was slated for demolition, and the collapse may have been the result of an accident related to that project, though authorities have not yet confirmed the direct cause. This post will be updated.

Update, 2:09 p.m.: Rescuers are still working to free at least one person trapped under the rubble. The Fire Commissioner told reporters that his team had located the victim and assured reporters "we're going to continue until we can get them out and we can get them to hospitals."

According to Fox News, rescue crews are running a low-tech operation. They report:

Firefighters were conducting a "hands-only" operation, with no heavy equipment for about the first hour. It was firefighters with buckets moving bricks. A team of four search dogs are also deployed at the scene.